Knowledge of formation properties behind the casing can be beneficial for the optimization of production in oil and gas wells. Specifically, near production zones, information about the precise location of the water-oil contact surface is useful for optimal management of the resource with significant economic consequences. The integrity of the cement seal is also crucial to ensure zone isolation and avoid water production or contamination.
Conventional monitoring methods use resistivity measurements to determine formation characteristics behind the casing. Several tools for behind the casing resistivity have been proposed, without significant commercial success so far, due to a significant uncertainty of the results. For example, due to the highly conductive nature of the metal casing, it is difficult to monitor the resistivity of the formation and cement. In some cases, more than one pipe may be present, thus making it impossible to use through the casing resistivity measurements.